The Senate moved to the verge of passing landmark immigration legislation yesterday, scheduling a final vote for today on a bill that would tighten the borders while allowing millions of illegal immigrants to stay in the country and permitting new guest workers to come and go.

But the very mixture of get-tough and be-kind measures that have made Senate approval possible could prove the biggest obstacle to reaching an accord with the House, where conservatives are determined to secure the borders before tackling other matters. Senate leaders said their coalition is fragile, and it may be hard pressed to survive changes that House members signal they will demand this summer.

"There are plenty of things wrong with this bill, but there are plenty of things right with it," Sen. Richard J. Durbin (D-Ill.) said yesterday, acknowledging the hodgepodge of compromises built into the Senate measure that leave virtually no one enthusiastic about it without reservation.

The Senate voted 73 to 25 to prevent endless debate on the legislation, setting up a final vote for today. Proponents called yesterday's "cloture" vote a bipartisan victory and a momentum-builder that virtually ensures passage of the bill today. But some of the measure's harshest critics voted for cloture, suggesting the final margin may be closer.

"We fought off a number of very cleverly crafted amendments that would have basically destroyed the bill," said Sen. John McCain (R-Ariz.), a leader of the Senate effort. But several battles over amendments, from the political left and right, involved close votes in recent days, underscoring what Sen. Arlen Specter (R-Pa.) has called "the fragile, delicate coalition that we have for this bill."

Anticipating the clash with the House, White House political adviser Karl Rove met privately with House Republicans yesterday, urging them to move closer to the Senate position, which President Bush embraces. He got a cold reception, according to people who attended. In particular, they said, Rep. John Carter (R-Tex.) testily told Rove that he was underestimating Americans' hostility to the notion of "amnesty" for illegal immigrants, especially if there is not proof that tougher border measures are taking effect. Rep. Sue W. Kelly (R-N.Y.) also told Rove that the president's position was a tough sell in her district, the sources said.

The House bill, passed in December, deals only with border and workplace enforcement matters. The Senate bill addresses such matters, calling for a 370-mile fence along the Mexican border, 6,000 National Guard troops to support border agents, aerial surveillance, road construction to aid border patrols, and other items.

But the Senate bill also calls for a guest-worker program and a three-tiered system for dealing with the estimated 11 million to 12 million illegal immigrants in the country. Those here five years or longer would be allowed to stay and apply for citizenship, provided they pay back taxes, learn English and have no serious criminal records. Those here two to five years would eventually have to return to a point of entry in Mexico or Canada and apply for a green card, which could allow their immediate return. The roughly 2 million immigrants who have been in the United States illegally for less than two years would be ordered home.

Even its proponents privately agree that the three-tiered plan is an ungainly compromise that resulted from long negotiations designed to build and hold a centrist coalition in the Senate. The "touchback" requirement for the middle category has especially drawn questions about its usefulness. Defenders say it will force immigrants to undergo background checks and is symbolically important to lawmakers who want immigrants to enter the country under legal circumstances.

Sen. Dianne Feinstein (D-Calif.) said this week that she does not believe "it is realistic to assume that, first, the Department of Homeland Security is going to be able to go out and deport 2 million people, and then secondly, to ensure that the other 2.8 million leave to go back for the 'touchback' program." But supporters of the Senate bill rejected her bid to scrap the three-tiered system, saying it threatened to fracture their coalition.

What senators are gingerly assembling, however, is unacceptable to a significant number of House Republicans, several lawmakers said.

"The majority of the people that I know, I don't think we are going to bend at all on this amnesty that is known as a 'pathway to citizenship,' " Rep. Walter B. Jones Jr. (R-N.C.) said in an interview. If Bush had cracked down on border security four years ago, he said, the House might consider the Senate's broader measures.

Jones and others predict that House negotiators will insist on significant changes to the Senate version in the conference committee, which is expected to begin work in June. But such changes could fracture the Senate's coalition, making passage of a final compromise difficult or impossible.

"I can't be for the bill without the balance it has now," Sen. Joseph R. Biden Jr. (D-Del.) said yesterday. The three essential elements, he said, are greater border security, a guest-worker program and a system for dealing with illegal immigrants similar to the three-tiered scenario.

A frequent topic of discussion in Capitol hallways is how an impasse over immigration might affect the November midterm elections. Some Democrats say voters will view Republicans as ineffective, because they control the House, Senate and White House. Some Republicans predict that Senate Democrats can be blamed if they spearhead a rejection of whatever emerges from the House-Senate conference.

Sen. Lindsey O. Graham (R-S.C.) warned both parties yesterday to abandon such thoughts. "To those who believe that no bill is a good answer: You're dead wrong," he told reporters. "The American people know what the answer should be, it's within the body of this bill. . . . To do nothing is a political loser."